The present invention relates to a cathode, and more particularly, to a photocathode particularly for use in an image intensifier and having a short wavelength infrared response.
Photocathode devices are optic electronic detectors which employ the photo emissive effect to respond to light. When photons impinge on the surface of the cathode, the impinging photons cause electrons to be emitted from the cathode. Many photocathode devices are made from semiconductor materials, such as gallium arsenide (GaAs). While GaAs is preferred, it is noted that other III-V compounds can be used such as, gallium phosphide (GaP), gallium indium arsenide phosphide (GaInAsP), indium arsenide phosphide (InAsP), as well as others. Essentially, visible light and near infrared (NIR) cathodes based on gallium arsenide materials have been available for many years. These cathodes in conjunction with microchannel plates (MCPs) and phosphor screens are utilized in high efficiency light amplification systems. These systems are employed in state of the art image intensifier tubes or devices. Such devices are utilized by the military and in many other applications. In addition to extremely high efficient light amplification, the resolution of these systems is greater than that of pixilated designs based on charge coupled devices (CCDs) and CMOS based sensors (APS). There is a great deal of investigation in regard to lower light level solid state image sensors, based on silicon technology. This work is continuing and strives to advance the spectral range of detection further into the NIR compared to gallium arsenide based technology. Pixilated designs and other material systems, in order to directly extend the spectral range are being proposed, but they do not have the resolution compared to current silicon technologies.
Many advances are being made in detector technology regarding silicon readout and the state of the art is improving, especially in the area of microbolometers. This pushes the spectral range further to the mid and far infrared wavelengths. In any event, this technology does not offer an alternative for producing high resolution, direct or indirect view options, with the spectral range in the short wavelength infrared (SWIR) portion of the electromagnetic spectrum.
The present invention depicts a cathode design capable of imaging the SWIR region of the electromagnetic spectrum, while retaining the advantages of the gallium arsenide based technology utilized in modern image intensifiers. The resulting technology can be used in a direct view system or coupled to a commercial CCD device to provide a versatile SWIR system as a SWIR intensified CCD. Previous attempts at extending the spectral range of image intensifiers relied on a direct substitution of the cathode materials. For example, silicon was substituted for GaAs or InGaAs for GaAs.
In any event, in certain instances the substitutions provided acceptable negative infinity devices (Si or InP to a lesser degree). However, in the case of silicon, an unacceptable cathode thickness is required to absorb the radiation due to the indirect band gap of the material. The increased thickness leads to electron spreading due to the diffusion and this reduces the overall image characteristics of the device. If one substitutes GaAs with InGaAs based compounds, one achieves a lower overall negative electron affinity characteristic. This characteristic is so low, that the photo response becomes negligible at high Indium concentrations. This effect may be due to the narrow band gap of the material in conjunction with the high electron affinity, or may just be due to the stoichiometry of the cesium oxide layer at the emission surface. In any event, direct substitution of the cathode material is not and has not been very successful.
The prior art concerning photocathodes show a wide variety of various techniques for extending the spectral range of the cathodes by utilizing multi-layer heterojunctions to compensate for the thickness, band gap, electron affinity, and activation characteristics of the different SWIR materials. Two of these SWIR cathode concepts that are disclosed are based on InP/InGaAs materials and transferred electrons. See for example, U.S. Pat. No. 5,047,821 issued to Costello, Spicer and Aebi in 1991. See also U.S. Pat. No. 6,121,612 issued to Sinor, Estrera and Couch in 2000. In both instances, the InGaAs is grown lattice matched to the InP material which is used as the emission material. In these instances, there is a compromise between electron affinity for material quality. By lattice matching the material, the interface between the InP and the InGaAs is of high quality, leading to low dislocation density and low recombination centers.
The lattice-matched material has a discontinuity in the conduction band which operate to block electrons from flowing from the narrow gap material into the emission material. To compensate for this the bias on the device must be large enough for the electrons to be thermionically emitted over the barrier. The required bias also introduces a field in the narrow gap material leading to enhanced recombination, mitigating some of the advantages of growing on lattice-matched materials. One other factor in common between cathode structures in the above-noted patents, is the formation of the emission contact. In both cases, the recommended emission surface contact is the cesiated silver layer. The silver is included to provide conductivity to bias the structure, while the cesium allows emission of electrons into a vacuum. A disadvantage of this layer is that photo generated electrons will not be blocked from entering the silver layer. These electrons are thus lost to the external circuit, and are not emitted to the vacuum for signal formation. While certain cathodes, as described in the above-noted U.S. Pat. No. 5,047,821, are commercially available, they exist only in an active configuration. There are other references which portray methods of adding biasing contacts on the emission surface of standard GaAs cathodes rather than the cesiated silver. For instance, layers of TiW overcoated with SiN have been used to provide addressable NEA cathode structures. The photocurrent is modulated by applying a voltage to the control electrodes.
In any event, the contact is operative to turn off electron emission rather than enhance it. As can be seen, the technique requires the deposition of a thick metal directly on the emission surface of the GaAs. Since the metal was in direct contact with the GaAs, the dark current of the cathode is relatively high and photo-generated carriers are lost to the metal.
In contrast, examination of U.S. Pat. No. 6,069,445 issued on May 30, 2000, to Arlynn W. Smith, one of the inventors herein, and is assigned to ITT Industries, Inc., the assignee herein. This patent depicts a photocathode device for use in an image intensifier of a night vision device. The photo emissive wafer includes a first contact disposed on a peripheral surface for electrically contacting the wafer. An annular shaped second contact is disposed on the emission surface of the wafer for enabling a potential difference to be applied across the wafer to facilitate the emission of photo-generated carriers from the emission surface. The active layer consists of GaAs doped to a concentration level of between 1xc3x971017 cmxe2x88x923 and 5xc3x971017 cmxe2x88x923 with the window composed of AlGaAs. In the U.S. Pat. No. 6,069,445 device, the photo-generated carriers are prevented from entering the second contact region of the device by the large blocking barrier provided by leaving the etch stop layer of the AlGaAs in place. The energy barrier created by the etch stop layer limits the dark current in the cathode to thermionic emission over the barrier. Therefore, photo-generated electrons are pushed towards the emission surface by the internal electric field created by the bias potential, but cannot enter the contact due to the large barrier from the material discontinuity. In this case, signal electrons are not lost to the bias contact as is the case for cesiated silver. Thus, the use of an AlGaAs blocking contact is described in U.S. Pat. No. 6,069,445.
It is therefore an object of the present invention to provide a novel cathode design, which expands the spectral range of current image intensification systems to the short wavelength infrared (SWIR) range of electromagnetic spectrum.
A novel cathode design is shown, which extends the spectral range of current image intensification systems to the short wavelength infrared (SWIR) range of the electromagnetic spectrum which is between 1.0 to 1.75 xcexcm. The cathode design and structure has a high emission probability. The cathode structure is a heterojunction of GaSb and GaAs, the GaSb material is to absorb the radiation and the GaAs is for emission characteristics. Each layer has a doping profile, which are used to minimize the effects of band offsets at the heterojunction and to provide a nearly flat conduction band profile throughout the cathode structure. The condition of nearly flat conduction band is supplemented by the use of an additional contact at the emission surface of the cathode where a bias is applied. The use of insulating technology prevents photo-generated signal electrons from entering the contact, thereby assuring their emission from the surface and operative to maintain low dark current characteristics for the intensifier cathode. The resulting SWIR image intensifier has all the advantages of current image intensifiers in terms of resolution and gain, but has a low dark current characteristic.